If two strongly religious gay people believe that they shouldn't be married because of their religious beliefs, I would be a little sad, but it would be fine for them to do that. If gay people began forcing religious people to have gay sex with each other, I would think it was wrong. But if all you have is an opinion, you can't go interfering with other people's lives. Again, fine for Christians and Muslims to not believe gay people marrying is okay, not fine for them to enforce those beliefs on others.